The present invention relates to a developer and a color developer for developing an electrostatic latent image in electrophotography, electrostatic recording, electrostatic printing and so on. The present invention also relates to an image forming apparatus for forming an image using the developer.
In electrophotographic image formation, charged toner particles are attached to a latent image formed on an image carrier of a photoconductive material to form a visible toner image, which in turn is transferred onto a recording medium such as a sheet of paper and fixed into an output image.
In recent years, the technology of copiers and printers is rapidly shifting from monochrome to full color, and the market for full-color devices is expanding. In full-color electrophotographic image formation, all colors are reproduced by overlaying toner images of the primary colors, which are cyan, magenta, and yellow, or toner images of the three colors and black. Thus, in order to obtain a sharp color image with high color reproduction, the surface of a fixed toner image must be relatively smooth to reduce scattering of light. This is the reason that many of conventional full color copiers or the like are designed to produce a relatively high gloss of 10 to 50%.
In general, fixation of a dry toner image on a recording medium is carried out by a contact-heat fixing method in which a roller or belt having a smooth surface is heated and pressed against the toner image. This method is thermally efficient and permits high-speed fixation, and can provide gloss and transparency to color images. However, since the surface of the heat fixing member is brought into pressure contacted with toner in a molten state and removed therefrom, a so-called offset phenomenon occurs in which part of the toner image adheres to the surface of the fixing roller and is transferred onto another image.
For the purpose of prevention of the offset phenomenon, a method has been widely adopted in which a releasing oil such as a silicone oil is applied to a surface of a fixing roller made of a material with high releasing properties such as a silicone rubber or a fluororesin. This method is very effective to prevent offset of toner, but needs a device for supplying a releasing oil and thus increases the size and cost of the fixing unit. Thus, in monochrome toners, the viscoelasticity of toner in a molten state is increased by, for example, controlling the molecular weight distribution of the binder resin to prevent internal breakage of the molten toner and a releasing agent such as a wax is added in the toner so that fixation can be made without or with a small amount of releasing oil.
In color toners, however, the viscoelasticity of toner in a molten state must be decreased to smooth the surface of a fixed image in order to improve color reproduction. Thus, color toners are more likely to cause offset than monochrome toners and cannot be fixed without application of releasing oil to a fixing roller. Also, when a releasing agent is added in a toner, it increases adhesion of toner, deteriorating the transferability of the toner. In addition, the releasing agent in the toner contaminates the triboelectrifying members such as carrier, lowering the chargeability and duration thereof.
Conventionally, low-molecular weight binder resins with which gloss can be easily obtained, such as polyester resins and epoxy resins, are used in color toners. These resins, however, contain a hydrophilic group and thus cause fluctuations of the charge amount of the developer with changes of humidity. In recent years, the trend is toward small particle size toners for high-quality images. Polyester resins and epoxy resins are inferior in grindability to styrene resins conventionally used as binder resins for monochrome toners.
Under such circumstances, various types of toners have been proposed. For example, Japanese Laid-Open Patent Publication H08-220808 discloses a toner comprising a linear polyester resin having a softening point of 90-120° C. and a carnauba wax. Japanese Laid-Open Patent Publication H09-106105 discloses a toner comprising a resin and a wax which are compatible with each other and have different softening points. Japanese Laid-Open Patent Publication H09-304964 discloses a toner comprising a polyester resin and a wax each having a specific melt viscosity. Japanese Laid-Open Patent Publication H10-293425 discloses a toner containing a polyester resin having a softening point of 90-120° C., a rice wax, a carnauba wax and a silicone oil. Japanese Laid-Open Patent Publication H05-61242 proposes a wax-containing toner produced by polymerization. However, a toner which does not cause offset with a fixing method in which no or only a small amount of oil is applied to a fixing roller and which is excellent in transferability, duration, charge stability against humidity and grindability has not been obtained yet.
As for carriers, a coating layer with high hardness and strength of a suitable resin material is often provided on carrier particles for the purpose of prevention of filming of toner components on surfaces of the carrier particles, formation of uniform carrier particle surfaces, prevention of surface oxidation and lowering of moisture sensitivity, extension of the service life of developer, prevention of adhesion of carrier particles to a photoconductor surface, protection of a photoconductor from scratches and abrasion by the carrier particles, control of charging polarity, adjustment of charge amount and so on. For example, there are disclosed a carrier coated with a specific resin material (Japanese Laid-Open Patent Publication S58-108548), a carrier coated with a resin material containing various additives (Japanese Laid-Open Patent Publications S54-155048, S57-40267, S58-108549, S59-166968 and H06-202381, and Japanese Examined Patent Publications H01-19584 and H03-628), a carrier containing an additive present on surfaces of the particles thereof (Japanese Laid-Open Patent Publication H05-273789), and a carrier coated with a coating film containing conductive particles having a particle size which is larger than the thickness of the coating film (Japanese Laid-Open Patent Publication H09-160304). Japanese Laid-Open Patent Publication H08-6307 discloses the use of a composition mainly composed of a benzoguanamines-n-butyl alcohol-formaldehyde terpolymer as a carrier coating material. Japanese Patent No. 2683624 disclosed the use of a crosslinked product of a melamine resin and an acrylic resin as a carrier coating material.
However, none of the carriers are sufficient in duration and ability not to adhere to a photoconductor surface. As for the duration, there are problems of so-called “spent phenomenon”, in which a toner film is formed on the surface of the carrier particles, and resulting fluctuation of charge amount, reduction of coating layer due to flaking of the coating resin and resulting lowering of resistivity and so on. Although high quality images can be obtained at an early stage of copying, the image quality decreases with increase of the number of copies produced. There is still room for improvement.